Light and electron microscopic examination of endomyocardial tissue obtained by catheter biopsy from the right side of the ventricular septum constitutes an useful and accurate technique for evaluating the degree of cardiac toxicity produced by the administration of antineoplastic agents of the anthracycline family. These agents produce characteristic myocardial lesions, in which two main components are recognizable: myofibrillar loss and cytoplasmic vacuolization due to dilatation of the sarcoplasmic reticulum. When toxicity is minimal, these changes affect only a small percentage of the myocytes; as the severity of the toxicity increases, a progressively larger percentage of myocytes is affected. Accordingly, a semi-quantitative scale (Billingham scale) has been devised in which these changes are graded from 0 to 3+. The grading score has been found to correlate with the total cumulative dose of the anthracycline, and with the extent of cardiac functional impairment. High scores have been found to be associated with risk factors such as cumulative doses of anthracyclines in excess of 550 mg/m2, advanced age (> than 70 years of age), systemic arterial hypertension, presence of other cardiovascular diseases, and previous radiotherapy to the mediastinum. In patients with these risk factors, monitoring by cardiac biopsy is recommended to guide therapy with anthracyclines beyond the empirical dose limitations.